There is no standard formula that applies to all measurements involving linear feet. The conversion depends upon what is being measured. For example, to convert 12 linear feet of files boxes into cubic feet, you would also need to know the width and height of the file boxes. Assuming that they are 1.25 feet wide and 1 foot tall, then 12 linear feet of them would be 15 cubic feet of file boxes (1 foot times 1.25 feet times 12 feet).